Dual-walled drill pipe used for drilling bore holes in earth formations are well known. Generally, bore holes are formed by rotating or percussively-rotating a drill string, which may be up to several hundred feet in length, into an earth formation using a drilling apparatus or rig. The drill string is comprised of an outer pipe string, to the lower end of which is connected a drill bit, and an inner pipe string. The inner and outer pipe strings together define an annular passageway for communicating fluid, such as air, to the bottom of the bore hole while the inner pipe string defines a bore which serves to return the air and cutting bits to the surface.
The outer pipe string is formed by a plurality of serially, threadedly connected lengths of pipe. The outer pipe string serves to transfer rotary and/or percussive forces to the cutting or drill bit and absorb axial forces such as those imposed when extracting the drill string from the bore hole. The axial forces may be substantial depending upon the length of the drill string. Accordingly, the outer pipe string is a high strength assembly which must be designed to withstand these loads.
The inner pipe string is also formed by a plurality of serially, connected lengths of pipe, although not threadedly connected. The primary function of the inner pipe string is to define the two above mentioned fluid passageways. It need not transfer rotary or percussive forces to the drill bit and, accordingly, need not meet the high strength requirements of the outer pipe string. Thus, it may be constructed of thinner-walled tube.
In order to minimize the strength requirements and, hence the size of the inner pipe, it is known to premount and secure an inner pipe to an associated outer pipe so as to form a dual-wall drill pipe section or assembly. In this manner, the weight of each inner pipe is transferred directly to its adjacent outer pipe. A drawback of this arrangement is that if either the inner or outer pipe is damaged, it is difficult to separate the two lengths of pipe so as to replace the damaged pipe. On the other hand, if the inner pipe is simply vertically supported within the outer pipe, the assembly may be difficult to handle prior to erecting or subsequent to dismantling a drill string because of the tendency of the two pipes to telescopically slide with respect to one another.
There is a need, therefore, for a dual-wall drill string section or assembly in which the inner pipe member is removably mounted within the outer pipe member so as to facilitate separation of the two pipe members in the event that one of the two members is damaged and must be replaced and yet positively retained therein to facilitate handling. The assembly must also be arranged so that no rotary forces are transmitted from the outer pipe member to the inner pipe member during drilling.